Environ Sci Pollut Res (2016) 23:11330–11338
DOI 10.1007/s11356-016-6351-8
RESEARCH ARTICLE
Anomalous abundance and redistribution patterns of rare earth
elements in soils of a mining area in Inner Mongolia, China
Lingqing Wang 1 & Tao Liang 1
Received: 1 December 2015 / Accepted: 25 February 2016 / Published online: 2 March 2016
# Springer-Verlag Berlin Heidelberg 2016
Abstract The Bayan Obo Mine, the largest rare earth element
(REE) deposit ever found in the world, has been mined for
nearly 60 years for iron and rare earth elements. To assess the
influences of mining activities on geochemical behavior of
REEs in soils, 27 surface soil samples and three soil profile
samples were collected from different directions in the vicinity
of the mine area. The total concentrations of REEs in surface
soils varied from 149.75 to 18,891.81 mg kg−1 with an average value of 1906.12 mg kg−1, which was apparently higher
than the average values in China (181 mg kg−1). The order of
the average concentrations of individual REEs in surface soils
was similar to that in Bayan Obo ores, which confirmed that
the concentration and distribution of REEs in the soils was
influenced by the mining activities. The concentrations of single REE in the soil profiles showed a similar trend with depth
with an increase at 0–25 cm section, then decreased and
remained relatively stable in the deep part. The normalized
curves inclined to the right side, showing the conspicuous
fractionation between the light and heavy REEs, which supported by the North American Shale Composite (NASC) and
Post-Archean Australian Shale (PAAS) normalized concentration ratios calculated for selected elements (LaN/YbN, LaN/
SmN, GdN/YbN). Slight positive Ce anomaly and negative
Eu anomaly were also observed.
Responsible editor: Zhihong Xu
* Lingqing Wang
[email protected]
* Tao Liang
[email protected]
1
Key Laboratory of Land Surface Pattern and Simulation, Institute of
Geographical Sciences and Natural Resources Research, Chinese
Academy of Sciences, Beijing 100101, China
Keywords Rare earth element . Soil . Bayan Obo Mine .
Enrichment . Mining activities
Introduction
Rare earth elements (REEs) are a group of 15 elements of which
only promethium (Pm) does not occur naturally in the Earth’s
crust (Liang et al. 2014). REEs share similar chemical and physical properties and tend to exist together naturally rather than in
isolation, which explains their very similar behavior in the environment (Tyler 2004; Wang et al. 2011). REEs can be divided
into two groups by their atomic number and masses—light
REEs (LREEs) from La to Eu and heavy REEs (HREEs) from
Gd to Lu. The REE abundance in the Earth’s crust varies from
0.5 mg kg−1 (Tm) to 66 mg kg−1 (Ce) with a mean of about
215 mg kg−1 for total REEs (Tyler 2004).
Human activities such as mining and construction activities
can significantly disturb the natural flow of elements between
major ecosystem reservoirs and alter the physical and chemical
processes at the Earth’s surface (Sen and Peucker-Ehrenbrink
2012). It is widely recognized that mining activities for rare earth
ores have changed the environmental conditions, causing serious environmental problems such as permanent damages to
ecosystems, severe soil erosion, air pollution, and loss of biodiversity (Salomons 1995; Aguilar et al. 2004). Meanwhile, the
large-scale and rapid increases of the exploitation activities of
REE resources have resulted in substantial increases of REE
levels in soils around the mining area. Rare earth elements exist
in a variety of minerals distributed in a wide variety of mineral
classes including halides, carbonates, oxides, phosphates, and
silicates (Jordens et al. 2013; Liang et al. 2014). Generally,
low concentrations of REEs are found in environment and they
can accumulate following anthropogenic inputs because of the
low mobility of these elements (Cao et al. 2000). Rare earth
Environ Sci Pollut Res (2016) 23:11330–11338
element abundances in soils around the mining areas are influenced by their parent materials, texture, weathering history and
pedogenic processes, organic matter contents and reactivity, and
anthropogenic disturbances (Aubert et al. 2002; Tyler 2004;
Morgan et al. 2012).
Rare earths are widely used for traditional sectors including
metallurgy, petroleum, textiles, and agriculture, and they are also
becoming indispensable and critical in many high-tech industries such as electronic products, green energy, and aerospace
alloys (Brioschi et al. 2013). The global demand for REEs is
growing at a rate of 3.7–8.6 % annually (Tan et al. 2015). In
2011, China produced over 90 % of the world’s rare earth supply
with only 23 % of the world total reserves (Anonymous 2012).
Due to the continuous exploitation of rare earths minerals, the
percentage of China’s rare earth reserve in the world has been
steadily decreasing from 75 % in 1970 to 23 % in 2011 (Chen
2011; Anonymous 2012). The presence of excessive REEs in
soils may act as pollutants causing serious consequences for
surrounding ecosystems, groundwater, agricultural productivity,
and human health (Li et al. 2013). Studies have indicated the
harmful effects and health hazards of REEs to human beings; for
example, the long-term exposure of REE dust may cause pneumoconiosis in humans (Hirano and Suzuki 1996).
REEs have caused widespread concern because of their
similar chemical behavior (Wang et al. 2011; Liang et al.
2014), their persistence and environmental behavior (Aubert
et al. 2002; Laveuf and Cornu 2009; Agnan et al. 2014), and
chronic toxicity (Brioschi et al. 2013; Li et al. 2013).
However, there is still lack of knowledge of REE accumulation and ecological effects in soils around the rare earth mining
areas. The Bayan Obo REE-Nb-Fe Mine is the largest currently existing REE deposit, which was discovered in 1927 and
started the production of rare earths in 1957. Bayan Obo Mine
is a surface mine and open-pit methods are used. REEs are
extracted from the minerals as a by-product of iron ore extraction. Outdated production processes and techniques in the
mining, dressing, smelting, and separating of rare earth have
severely damaged surface vegetation; caused soil erosion, pollution, and acidification; and reduced or even eliminated crop
output. In this study, REE distribution in soils around the
Bayan Obo Mine has been investigated to assess their pollution levels caused by the mining activities. The results will
provide scientific evidence for evaluating the ecological risks
associated with the exploitation of rare earth minerals and
preventing REE pollution in soils and eco-resumption.
Materials and methods
Study area
The Bayan Obo Fe-REE-Nb Mine (109° 59′ E, 41° 48′ N) in
Inner Mongolia is the world’s largest REE deposit. It is located
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in the northern margin of the North China Craton, and hosted
in the Mesoproterozoic Bayan Obo Group (Sun et al. 2013).
The Bayan Obo region has a semi-arid, temperate, and continental monsoon climate. The mean annual temperature is
2.4 °C ranging from −16.2 °C in January to 19.4 °C in July.
The average annual precipitation is about 240–400 mm, and
evaporation is 2100–2700 mm. The prevailing wind direction
is northwest. The average annual wind speed is 5.5 m/s and
the average number of days with strong wind is 70 days per
year.
The most prominent feature of the Bayan Obo Mine is that
the ore-bearing dolomite marbles, which extend about 16 km
long from west to east and are 1–3 km wide, contain remarkably high concentrations of REEs. REEs occur dominantly as
monazite and bastnasite, but more than 20 REEs-containing
minerals and 15 niobium minerals have been reported (Wu
2008). Their total reserves have been estimated as at least
1.5 billion tons of iron (average grade 35 %), at least 48 Mt
of REOs (average grade 6 %), and about 1 Mt of niobium
(average grade 0.13 %) (Drew et al. 1990). The Bayan Obo
Mine produced 55,300 t of RE2O3 in 2005, accounting for
47 % of the total rare earth production of China and 45 % of
that of the world (Wu 2008). The dolomite marble contains
many iron ore bodies, scattering from west to east and occurring as large lens or beds as individual ones. They may be
divided into three portions. The biggest ones are the main
ore body and the eastern ore body (Fig. 1), each of which
include iron-REE resources with more than 1000 m of strike
length and average 5.41 and 5.18 % rare earth oxides (REOs),
respectively (Yuan et al. 1992). The Western ore body is composed of many small sub-ore bodies with exploitations on a
small scale (Xu et al. 2008).
Sampling and measurement
Surface soil samples were collected at a depth of 0–10 cm at
27 sites located in the vicinity of the Bayan Obo Mine (Fig. 1).
According to the position of the mining area and prevailing
wind direction, these sites were divided into four areas: southeast (labeled An, n = 1, 2, …, 8); southwest (labeled Bn, n = 1,
2, …, 8); northwest (labeled Cn, n = 1, 2, …, 5); and northeast
(labeled Dn, n = 1, 2, …, 6). Three soil profiles (labeled Pn,
n = 1, 2, 3) up to a depth of 70 cm were also selected to reveal
the vertical distribution of REEs. A total of 11 sections at
depths of 0–3, 3–6, 6–9, 9–12, 12–15, 15–20, 20–25, 25–
30, 30–40, 40–50, and 50–70 cm were taken from each soil
profile. All samples were stored in a portable cryostat and
transported to the laboratory. The collected soil samples were
frozen-dried and sieved through a 100-mesh sieve to remove
large debris, stones, and pebbles prior to analysis. The main
soil type is chestnut soil (Haplic Krastazem, FAO) in the
Bayan Obo area and it has a pH value of 7.8, organic matter
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Environ Sci Pollut Res (2016) 23:11330–11338
Fig. 1 Map of the study area and sampling sites
content of 2.03 %, total P and Olsen P of 722.61 and
27.656 mg kg−1, respectively.
Soil samples (0.05 g) were digested in a mixture of
3 ml HNO3, 0.5 ml HClO4, and 6 ml HF in a sealed
Teflon vessel on hotplate at 200 °C for 8 h. The digested
solution was evaporated to near dryness. The residue was
dissolved by 2 % HNO3 and transferred into a 50-ml
flask, then diluted to volume with deionized water. The
blank and duplicate digest were carried out in the same
way. REE concentrations in the digested solutions were
measured using inductively coupled plasma mass spectrometry (ICP-MS, ELAN DRC-e, Perkin Elmer SCIEX)
(Wang et al. 2011). The detection limit of ICP-MS is
1 ng l−1. All measurements were carried out in duplicate.
The quality control was achieved using certified reference
samples GBW07303 from the National Research Center
for Certified Reference Materials (Beijing, China). The
results for the reference samples were in line with the
reference values, with the deviations less than 5 %.
Enrichment factor (EF) was calculated based on the normalization of analytical data against the reference element and
widely used to estimate the anthropogenic impact on soils
(Sutherland 2000; Loska et al. 2004; Tang et al. 2010). The
EF is defined by the following formula:
E F ¼ ðC i =C r Þsample =ðC i =C r Þcrust
where Ci is the concentration of the element considered
in the studied samples or the crust, and Cr is the concentration of reference element in the studied samples or
the crust. A reference element should not be influenced
by anthropic activities and should be little affected by
weathering processes. The most common reference elements used in literature are Al, Ca, Fe, Li, Mn, Sc, and
Sr (Sutherland 2000; Tang et al. 2010). In this study, Al
was used as a conservative element to calculate the EFs
of REEs. Contents of REEs and Al in the upper continental crust were taken from McLennan (2001).
Contamination categories can be identified on the basis
of the enrichment factor. A result of EF < 2 indicates
deficiency to minimal enrichment, 2 < EF ≤ 5 means
moderate enrichment, 5 < EF ≤ 20 means significant enrichment, 20 < EF ≤ 40 signifies very high enrichment,
a nd E F > 40 m e an s e xt r e m e l y h i g h e nr i c h m e nt
(Sutherland 2000).
Environ Sci Pollut Res (2016) 23:11330–11338
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Results and discussion
Descriptive statistics of REE concentrations in soils
The basic statistics of REE concentrations in the surface soil
samples were summarized in Table 1. The coefficients of variation (CVs) of La, Ce, Pr, Nd, Sm, Eu, and Gd were greater
than 100 %, showing strong variability. The variability of Tb,
Dy, Ho, Er, Tm, Yb, and Lu was moderate with the CV fluctuated from 24.1 to 95.4 %. Low CV values indicated a spatially homogeneous distribution of REE concentrations,
whereas high CV values indicated a non-homogenous surface
distribution in the study area (Karanlik et al. 2011).
The Kolmogorov-Smirnov (K-S) test was applied to determine if the data followed a normal distribution. The results
indicated that REE concentrations were not normally distributed (p < 0.05) except for Er, Tm, Yb, and Lu. As a result, the
geometric mean or the transformed mean (log transformed or
Box-Cox transformed) are used to describe average concentrations (McGrath et al. 2004). In the present study, following
a Box-cox transformation, all the data successfully passed the
K-S test for normality.
Strong variation was observed in the total REE concentrations among the surface soil samples around the Bayan Obo
region, which varied from 149.75 to 18,891.81 mg kg−1 with
an average value of 1906.12 mg kg−1. The sum of all REEs
(∑REE) in the study was much higher than those reported by
Hu et al. (2006) with a mean REE concentration in China of
181 mg kg−1 (1225 soil samples), and it was also higher than
the background values of soils in the Bayan Obo region (Li
et al. 2010). Other authors have found slightly lower
Table 1
concentrations of REEs in Japan (98 mg kg−1, Yoshida et al.
1998); Australia (105 mg kg−1, Diatloff et al. 1996); and
Germany (305 mg kg−1, Loell et al. 2011). The arithmetic
means, geometric means (GMs), and medians of REEs in
the surface soils are higher than their background values in
soils of Bayan Obo region (CNEMC 1990).
Results from the present study revealed a highly variable
concentration of individual REEs, in the following decreasing
order: Ce > La > Nd > Pr > Sm > Gd > Dy > Er > Yb > Eu >
Tb > Ho > Tm > Lu. The sequence was similar to that in
Bayan Obo ores, determined in 2012 (Xu et al. 2012), and
similar to the order of REEs in the sediments (Zhu et al.
1997), animal, plant, environmental, and geological samples
(Wang et al. 1995). The concentrations of LREEs accounted
for the main part with 83~95 % of the total, while cerium
accounted for 48 %. Similar trend was observed in the
Bayan Obo ores, which indicated that the concentration and
distribution of individual REE was influenced by the mining
activities. The concentrations of single REEs conformed to the
distribution law of odd-even numbers (Wang et al. 2011).
Meanwhile, there is a statistically significant correlation
(R2 > 0.89; p < 0.05) between all the elements examined.
This suggests a similar source of these elements caused by
the rare earth mining. Previous studies have reported that
REEs can continuously accumulate in surface soils following
various pathways such as atmospheric deposition (Tyler
2004), mining activities (Zhang et al. 2000), and application
of REE fertilizers (Pan et al. 2002) because of the low mobility
of these elements. Results in the present study suggest that soil
environment in Bayan Obo mining area was seriously polluted from the industries involving rare earth exploitation and
Descriptive statistics of REE concentrations (mg kg−1) in surface soils
Mean
SD
CV (%)
Min
Max
GM
Q1
Q2
Q3
Background valuea
K-S p
La
Ce
Pr
Nd
Sm
Eu
Gd
Tb
Dy
Ho
Er
Tm
Yb
518.14
982.78
88.81
262.63
20.77
2.11
15.43
1.46
6.37
1.04
3.29
0.38
2.56
1101.49
1828.20
217.45
641.01
26.10
0.73
18.91
1.40
4.20
0.48
1.72
0.09
0.62
212.6
186.0
244.9
244.1
125.7
34.7
122.5
95.4
65.9
45.7
52.2
24.1
24.1
33.82
59.59
7.72
27.97
5.01
1.06
4.60
0.58
3.15
0.62
1.87
0.26
1.70
5421.58
8841.18
1035.59
3332.80
118.47
3.39
92.86
6.99
22.23
2.72
9.72
0.63
4.09
173.82
352.15
30.09
106.20
13.97
1.99
10.94
1.17
5.61
0.97
3.02
0.37
2.50
72.17
131.44
15.44
57.52
8.93
1.44
7.08
0.80
4.28
0.81
2.40
0.31
2.19
114.54
185.28
20.65
77.54
11.07
1.90
9.11
0.99
5.03
0.89
2.60
0.36
2.36
387.73
989.43
52.14
194.88
20.52
2.64
15.34
1.47
6.24
1.11
3.45
0.41
2.70
32.8
49.1
5.68
19.2
3.81
0.81
3.47
0.47
3.05
0.66
1.82
0.27
1.79
0.005
0.012
0.001
0.001
0.023
0.009
0.003
0.006
0.014
0.034
0.058
0.477
0.464
Lu
∑REE
0.35
1906.12
0.05
3804.00
13.2
199.6
0.27
149.75
0.42
18,891.81
0.35
727.79
0.33
310.55
0.35
433.24
0.38
1726.71
0.27
123.2
0.537
0.007
SD standard deviation, CV coefficient of variation, GM geometric mean, Q1 first quartile, Q2 median, Q3 third quartile
a
CNEMC (China National Environmental Monitoring Center), 1990
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production. A vast majority of the exogenous REEs are fixed
on solid surfaces and exist in inert forms, which are normally
concentrated in the top layer of soils around the mining areas.
Meanwhile, results of the present study revealed that the distribution patterns of REEs in soils are all in line with their
parent rocks. No obvious fractionation between REEs occurred during those transport processes.
Spatial distribution of REEs in surface soils
The experimental semivariogram of ΣREE concentrations could
be fitted with a spherical model with parameters: nugget effect
(C0) 1000 mg2/kg2; sill (C0 + C) 16,910,000 mg2/kg2; and range
3900 m. The ratio of nugget effect (C0) over sill (C0 + C), r = C0/
(C0 + C), can be used to express spatial correlations of REEs and
provide a quantitative basis for estimating points at unsampled
locations. If the ratio did not exceed 25 %, the REE concentrations were considered strongly spatially dependent; if the ratio
was between 25 and 75 %, the concentration was considered
moderately spatially dependent; and if the ratio was greater than
75 %, the concentration was considered weakly spatially dependent (Xie et al. 2012). In the present study, the ratio was smaller
than 25 %, showing strong spatial dependence.
The spatial distribution of ΣREE concentrations is a useful
aid to assess the possible sources of enrichment and to identify
hotspots with high REE concentrations. The estimated map of
REEs in soil is shown in Fig. 2. Several areas with high levels of
REE concentration were identified around the mining site in the
Fig. 2 Estimated map of REEs
concentration in surface soils
around the mining area
Environ Sci Pollut Res (2016) 23:11330–11338
map. REE concentrations were found to be highly concentrated
in the vicinity of the eastern ore and west ore areas and decreased
with the distance away from the central region of mining area.
This shows that the mining-milling operations and disposal of
tailings can contribute significantly to REE accumulation in
surface soils. It also showed a geographical trend, with high
REE concentrations in the southeastern area, which could be
caused by the strong northwesterly winds prevailed in this region. Wind erosion can be a major cause of the loss and dispersion of material from tailings impoundments into their surroundings because of the low precipitation and high evaporation rates
in the semi-arid climate areas (Razo et al. 2004). Under natural
conditions, the translocation ability of REEs is relatively weak
as compared with other metal ions (Liang et al. 2014). A vast
majority of the exogenous REEs are fixed on solid surfaces and
exist in inert forms, which are normally concentrated in the top
layer of soils around the mining area. It should also be noted that
the high REE concentration areas were generally found along
the railway. This may be attributed to the diffusion of rare earth
mineral powder during transportation process of rare earth minerals and tailings.
The average EF of the total REEs in the Southeast was
higher than 40, confirming the extremely high level of contamination of REEs in soils in this part; while the total REE in
the southwest and northeast directions, with the average EFs
between 5 and 20, were classified as significant enrichment.
The average EF of the total REEs in the northwest was relatively low and ranged between 2 and 5, indicating moderate
Environ Sci Pollut Res (2016) 23:11330–11338
enrichment. For the single REE, EFs can also be an effective
tool to differentiate a natural origin from anthropogenic
sources in this study. Mean values of EFs for La and Ce in
southeast directions were higher than 40, while Pr and Nd had
average EFs close to 40, indicating extremely high contamination of soils in these parts. Mean EFs ranged between 5 and
20 for La, Ce, Pr, and Nd in the southwest and northeast
directions implying significant influences of non-crustal
sources. Most of the HREEs, such as Ho, Er, Tm, Yb, and
Lu, had mean EFs less than 2, reflecting the lack of contamination with these elements. The EF analysis confirms that
REEs enriched in the soils with varying levels, which was
caused by anthropogenic sources and influenced by the prevailing wind in this region.
Vertical distribution of REEs in soils
The vertical distributions of REEs in three soil profiles from
different locations in the Bayan Obo Mine area are shown in
Fig. 3. The total concentrations of REEs in the three profiles
were different, with a minimum of 744.75, 874.96, and
1573.34 mg kg−1; a maximum of 1416.87, 1319.24, and
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1913.99 mg kg−1; and a mean of 1145.58, 1319.24, and
1740.46 mg kg−1, respectively. The distribution curves of
the total REE concentrations in the three profiles were also
different in pattern: The total REE concentration of soil profile
P1 increased with increasing depth from the surface to 10 cm
then decreased and remained relatively constant below a depth
of 20 cm (Fig. 3a). The total REE concentration of the whole
profile P2 was high and particularly elevated at 0–25 cm section, with a maximum at 20–25 cm in depth (Fig. 3b). The
total REE concentration of profile P3 was significantly elevated at 0–10 cm and maintained stability at 10–20 cm section,
but decreased significantly below a depth of 30 cm (Fig. 3c).
Figure 4 shows cumulative frequency distribution of REEs EF
in the soil profiles. The mean value of EF for the total REE
was 12.05 (ranging between 7.06 and 17.33), indicating a
significant enrichment of REEs in the soil profiles around
the mining area.
The concentrations of single REEs showed a roughly similar trend with depth, which increased at 0–25 cm section,
decreased, and remained relatively constant in the deeper part
(>25 cm). The EF values for La and Ce in all the soil profiles
were >5, indicating that they were significantly enriched in the
Fig. 3 Vertical distributions of REEs in three soil profiles (P1 (a); P2 (b); P3 (c)) from the different locations in Bayan Obo mining area
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Fig. 4 Cumulative frequency distribution of EF of REEs in the soil profiles
soil profiles. The EF values for Pr in 76 %, Nd in 77 %, Sm in
53 %, and Gd in 55 % in 2–5 reveal that these elements in
most of the soil profiles correspond to moderate enrichment.
The EF values of Dy in all soil profiles samples, Eu in 63 %,
Tb in 66 %, Tm in 68 %, Ho in 73 %, Er in 86 %, Yb in 95 %,
and Lu in 71 % were <2, revealing that these elements in most
of the soil profiles corresponded to minimal enrichment. The
enrichment and fluctuation of REE in soil profiles is mainly
attributed to the historical pollution from mining area and
anthropogenic disturbance.
Shale-normalized REE patterns
To eliminate the Oddo-Harkins effect and characterize the
REE patterns of soils, the estimated average composition of
REEs in the North American Shale Composite (NASC) and
Post-Archean Australian Shale (PAAS) (McLennan 2001)
were selected to normalize the measured REE concentrations
(Fig. 5).
It can be found that the two shale-normalized REE distribution patterns for soil were generally identical. The REE
patterns were characterized by LREE enrichment and HREE
depletion, with the curves extending downward from the left
to the right. The ratio of LaN/YbN was used to represent the
inclination of the shale-normalized curves. When the ratio of
LaN/YbN is greater than or equal to 1, the curves of LREE
incline to the right side, suggesting the soil is rich in LREE
and deficient in HREE. The ratio of LaN/YbN in the present
study was 6.97NASC and 4.70PAAS (Table 2). This means that
the soil samples around the rare earth ores belonged to a light
REE soil type intensified by the mineral exploitation.
Generally, higher concentrations of LREE were observed in
soils developed on phosphate and carbonate rocks, whereas
the basalt-weathered soils showed enrichment in HREE (Chen
and Yang 2010).
The distribution patterns of REEs in soil are characterized
by obvious fractionation of LREE and HREE. The LREEs
were more abundant in the soil samples and the
LREE/HREE ratio was 27.32. The normalized LREEN/
HREEN ratio was 3.93NASC and 2.94PAAS. The ratios of LaN/
SmN and GdN/YbN are used to measure the degree of LREE
fractionation and that of HREE fractionation, respectively.
The degree of LREE fractionation with La N /Sm N of
Fig. 5 NASC and PAAS normalized patterns of average REE
concentrations in surface soils
Environ Sci Pollut Res (2016) 23:11330–11338
Table 2
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NASC- and PASS-normalized patterns of REE concentrations in surface soils
LREE/HREE
27.32
LREE/∑REE
95.51 %
LaN/SmN
δEu
LaN/YbN
NASC
PAAS
NASC
PAAS
NASC
PAAS
NASC
PAAS
NASC
PAAS
NASC
PAAS
3.93
2.94
6.97
4.70
2.48
2.18
2.39
1.92
0.73
0.85
1.12
1.26
2.48NASC and 2.18PAAS was slightly higher than that of HREE
fractionation with (Gd/Yb)N of 2.39NASC and 1.92PAAS.
The depletion or enrichment of Ce and Eu usually occurred
in the natural environment, which may be linked to their oxidation state and their mobility under different oxidationreduction conditions (Semhi et al. 2009). Ce exists in soils in
both oxidation states, but under redox condition Ce3+ is more
easily oxidized to Ce4+ with higher oxygen fugacity, which is
much less mobile and results in positive Ce anomaly
(δCe > 1). In the case of Eu, it is an incompatible element in
its trivalent form (Eu3+) in an oxidizing magma and preferentially incorporated into plagioclase in its divalent form (Eu2+)
in a reducing magma, and this ion exchange process is the
basis of the negative Eu anomaly (δEu < 1). In the present
study, slight positive Ce anomaly with δCe of 1.12NASC and
1.26PAAS, and slight negative Eu anomaly with δEu of
0.73NASC and 0.85PAAS were also observed, indicating that
differentiation occurred between Ce, Eu, and other REEs in
the weathering processes of parent rock.
Conclusions
Influenced by the exploitation of rare earth mineral, REEs in
surface soils around the Bayan Obo Mine showed various
levels of enrichment. The concentrations of total REE varied
from 149.75 to 18,891.81 mg kg−1 with an average value of
1906.12 mg kg−1. The order of the average concentrations of
REEs in the surface soils around the Bayan Obo Mine area
was as follows: Ce > La > Nd > Pr > Sm > Gd > Dy > Er > Yb
> Eu > Tb > Ho > Tm > Lu. The sequence was similar to that
in Bayan Obo ores, which confirmed that the concentration
and distribution of REEs were influenced by the mining activities. REE concentrations were higher in the vicinity of the
eastern ore and western ore and decreased with the distance
away from the center of the mining area. The spatial variability of REE concentrations in soils were strong in the mining
area, which could be caused by the exploitation activities of
REE resources and the strong northwesterly winds prevailed
in this region. The concentrations of single REEs in the soil
profiles showed a similar trend with depth, which increased at
0–25 cm section, then decreased and remained relatively constant in the deep part. The degree of LREE fractionation with
LaN/SmN of 2.48NASC and 2.18PAAS was slightly higher than
that of HREE fractionation with (Gd/Yb)N of 2.39NASC and
GdN/YbN
δCe
LREEN/HREEN
1.92PAAS. Slight positive Ce anomaly with δCe of 1.12NASC
and 1.26PAAS, and slight negative Eu anomaly with δEu of
0.73NASC and 0.85PAAS were also observed. The spatial distribution of REEs in soils showed a strong gradient concentration along the prevailing wind direction. The normalized
curves inclined to the right side, showing the conspicuous
fractionation between the light REEs and heavy REEs.
Acknowledgments This study was sponsored by the National Natural
Scientific Foundation of China (nos. 41401591 and 41571473). Data
presented in this paper are available by direct contact with the first author
([email protected]).
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